University of Crete HEP Seminars


Recordings can be found in this link

Toy models for AdS black holes

Speaker: Adam Bzowski
Institution: University of Crete
Time: Tuesday 8 September 2020, 13:00
Venue: 2nd floor seminar room (webinar)
Abstract: I will present two simple toy models exhibiting properties associated with quantum black holes. For the first model I will consider a quantum mechanical system of the double well potential. The second model is obtained by coupling stress tensors of the two CFTs living on the boundaries of the BTZ black hole. The common theme of both systems is that the Hilbert space of states does not admit the usual factorization into a tensor product. Instead, the semi-classical, low-energy physics is emergent and can be reinterpreted in terms of state-dependence.
Recording: The recorded talk can be found here in BBB format and Mp4 format

Introduction to the large charge expansion

Speaker: Domenico Orlando
Institution: University of Torino
Time: Tuesday 15 September 2020, 13:00
Venue: 2nd floor seminar room (webinar)
Abstract: Working in sectors of large global charge leads to important simplifications when studying strongly coupled CFTs. In this talk I will introduce the large-charge expansion via the simple example of the O(2) model and apply it in a number of other situations displaying a richer structure, such as non-Abelian vector models, supersymmetric N=2 systems and asymptotically safe theories.
Recording: The recorded talk can be found here.

Quantum codes, lattices, and conformal field theories

Speaker: Anatoly Dymarsky
Institution: University of Kentucky
Time: Tuesday 22 September 2020, 13:00
Venue: 2nd floor seminar room (webinar)
Abstract: There is a deep relation between classical error-correcting codes, Euclidean lattices, and chiral 2d CFTs. We show this relation extends to include quantum codes, Lorentzian lattices, and non-chiral CFTs. The relation to quantum codes provides a simple way to solve modular bootstrap constraints and identify interesting examples of conformal theories. In particular we construct many examples of physically distinct isospectral theories, examples of "would-be" CFT partition function -- non-holomorphic functions satisfying all constraints of the modular bootstrap, yet not associated with any known CFT, and find theory with the maximal spectral gap among all Narain CFTs with the central charge c=4. We also discuss averaging over the ensemble of all CFTs associated with quantum codes, and its possible holographic interpretation. The talk is based on arXiv:2009.01236 and arXiv:2009.01244.
Recording: The recorded talk can be found here.

Turbulent hydrodynamics in strongly correlated Kagome materials

Speaker: Ioannis Matthaiakakis
Institution: Julius-Maximilians-Universitaet Wuerzburg
Time: Thursday 24 September 2020, 13:00
Venue: 2nd floor seminar room (webinar)
Abstract: In recent years a lot of interest has been directed towards realizing strongly-coupled electronic fluids. This interest is partly because a stronger coupling enables the transition to the hydrodynamic regime and partly because of the novel transport phenomena such a fluid promises. Strongly-coupled fluids are also a natural and fertile ground for testing and refining our understanding of AdS/CFT and holography in general. In this talk, I will first show that Kagome materials with a relativistic spectrum are a compelling platform for realizing strongly-coupled fluids. Then, I will employ holography to estimate their ratio of shear viscosity to entropy density. With that estimate at hand, I will finally argue that strongly-coupled Kagome fluids put for the first time non-linear hydrodynamic phenomena such as turbulence within experimental reach.
Recording: The recorded talk can be found here.

Tidal Trapping of Strings by Microstate Geometries

Speaker: Nicholas Warner
Institution: University of Southern California
Time: Tuesday 29 September 2020, 13:00
Venue: 2nd floor seminar room (webinar)
Abstract: I will describe the fate of a massless (or ultra-relativistic massive) string probe propagating down the BTZ-like throat of a microstate geometry in the D1-D5 system. Far down the throat, the probe encounters large tidal forces that stretch and excite the string. The excitations are limited by the very short transit time through the region of large tidal force, leading to a controlled approximation to tidal stretching. The amount of stretching is proportional to the incident energy, and that it robs the probe of the kinetic energy it would need to travel back up the throat. As a consequence, the probe is effectively trapped far down the throat and, through repeated return passes, scrambles into the ensemble of nearby microstates. This tidal trapping can lead to weak gravitational echoes.
Recording: The recorded talk can be found here.

Uncovering the Structure of the ε Expansion

Speaker: Andreas Stergiou
Institution: Los Alamos
Time: Tuesday 06 October 2020, 17:00
Venue: Webinar
Abstract: The ε expansion was invented almost 50 years ago and has been used extensively ever since to quantitatively describe critical phenomena. Its most famous applications are found in theories involving scalar fields in 4−ε dimensions. In this talk, we will be interested in the structure of the ε expansion of scalar field theories and the fixed points that can be obtained within it. Our motivation is based on the goal of classifying conformal field theories in d=3 dimensions. We will describe recently discovered universal constraints obtained within the framework of the ε expansion, focusing mostly on the 4−ε case although 3−ε will also be discussed. It will be shown that a “heavy handed" way to search for fixed points yields a plethora of new fixed points that reveal aspects of the structure of the ε expansion and suggest that a classification of conformal field theories in d=3 is likely to be very non trivial. (Based on arXiv:1707.06165, arXiv:1810.10541 and upcoming work with Hugh Osborn.)

The Inversion Formula and 3d CFTs

Speaker: David Meltzer
Institution: Caltech
Time: Tuesday 13 October 2020, 17:00
Venue: Webinar
Abstract: The Lorentzian inversion formula proves that CFT data is analytic in spin and that operators can be organized on Regge trajectories. In this talk we investigate the leading Regge trajectories of the O(2) model using a combination of numerical and analytical techniques. We find agreement between the two methods in various charge sectors of the O(2) CFT. Furthermore, we find evidence that scalar operators lie on Regge trajectories and estimate the Regge intercept of the O(2) model. Finally, we use the analytic and numerical data to study non-Gaussianities in the charge-1 scalar four-point function.
Recording: The recorded talk can be found here.

Symmetry, Confinement and Naturalness

Speaker: Zohar Komargodski
Institution: Simons Center
Time: Thursday 15 October 2020, 15:00
Venue: Webinar
Abstract: We will review the notion of extended symmetry and show how it can be applied for two dimensional QCD. We will use extended symmetry to prove deconfinement and to compute the exact string tension when the quarks are light. Some interesting implications for the naturalness of RG flows will be discussed as well.
Recording: The recorded talk can be found here.

AdS/CFT at Finite String Coupling

Speaker: Shai Chester
Institution: Weizmann Institute
Time: Tuesday 20 October 2020, 13:00
Venue: Webinar
Abstract: We study the N = 4 SU(N) super-Yang-Mills stress tensor multiplet four-point function at large N and finite complexified Yang-Mills coupling tau, which is dual to the Type IIB graviton correlator on AdS_5 × S^5 at large string length and finite string coupling. The specific four-point functions we consider are integrated correlators obtained by taking various combinations of four derivatives of m, b, and tau of the sphere free energy deformed by mass m and squashing parameter b, which can be computed using supersymmetric localization. We show that at each order in 1/N, these quantities can be written in terms of modular invariants, such as the well studied non-Holomorphic Eisenstein series as well as some new generalizations thereof. These results reproduce known features of the low-energy expansion of the four-graviton amplitude in type IIB superstring theory in ten-dimensional flat space, which is the first check of AdS/CFT at finite string coupling, and have interesting implications for the structure of the analogous expansion in AdS_5 × S^5.
Recording: The recorded talk can be found here.

Little String Theory on Curved Manifolds

Speaker: Ofer Aharony
Institution: Weizmann Institute
Time: Tuesday 27 October 2020, 14:00
Venue: Webinar
Abstract: This talk is based on 1908.02642, in collaboration with Evtikhiev and Feldman. After reviewing Little String Theories (the decoupled theories on the worldvolume of N NS5-branes) we will use their holographic duality to Type II string theory in asymptotically linear dilaton backgrounds in order to study these theories on curved space-times. We focus on backgrounds with a large number of Killing vectors (namely, products of maximally symmetric spaces), without requiring supersymmetry (we do not turn on any background fields except the metric). Little String Theory is non-local so it is not obvious which spaces it can be defined on; we show that holography implies that the theory cannot be put on negatively curved spaces, but only on spaces with zero or positive curvature. For example, one cannot put Little String Theory on a product of an anti-de Sitter space times another space, without turning on extra background fields. On spaces with positive curvature, such as products of spheres, we typically find (for large N) dual holographic backgrounds which are weakly coupled and weakly curved everywhere, so that they can be well-described by Type II supergravity. In some cases more than one smooth solution exists for Little String Theory on the same space, and they all contribute to the partition function. We also study the thermodynamical properties of Little String Theory compactified on spheres, finding the leading correction to the Hagedorn behavior of the spectrum, which is different on curved space than on flat space.
Recording: The recorded talk can be found here.

Inside the Hologram: The bulk observer's experience

Speaker: Lampros Lamprou
Institution: University of British Columbia
Time: Friday 06 November 2020, 17:00
Venue: Webinar
Abstract: I will present a holographic framework for reconstructing the experience of bulk observers in AdS/CFT. In particular, I will show how to recover the proper time and energy distribution measured along bulk worldlines, directly in the CFT via a universal, background-independent prescription. For an observer falling into an eternal AdS black hole, the proposal resolves a conceptual puzzle raised by Marolf and Wall. Notably, the prescription does not rely on an external dynamical Hamiltonian or the AdS boundary conditions and is, therefore, outlining a general framework for the emergence of time.
Recording: The recorded talk can be found here.

Humanly Traversable Wormholes

Speaker: Alexey Milekhin
Institution: Princeton University
Time: Tuesday 10 November 2020, 17:00
Venue: Webinar
Abstract: In my talk I will present the construction of humanly traversable wormholes in four dimensions. The construction is based on two magnetically charged black holes embedded into Randall-Sundrum model. In particular, it does not need exotic matter or boundary conditions. It is a long wormhole that does not lead to causality violations in the ambient space. I will explain why it is humanly traversable and what could be the challenges for the travelling observer. The talk is based on
Recording: The recorded talk can be found here.

Gravity: the force from the UV

Speaker: Simon Caron Huot
Institution: McGill University
Time: Tuesday 17 November 2020, 16:00
Venue: Webinar
Abstract: Dispersion relations are statements of causality. They give sum rules connecting infrared and ultraviolet physics. While they originate in optics and quantum field theory, I will argue that their 'true purpose' is gravity, viewed as a low-energy effective theory. This is due to the unique energy dependence of this force. I will present ongoing work mapping out the space of consistent effective theories, including recent results in scalar models (2011.02957), deriving a sharp version of dimensional analysis scaling from causality.
Recording: The recorded talk can be found here.

Asymptotics for the wave equation on black hole spacetimes

Speaker: Stefanos Aretakis
Institution: University of Toronto
Time: Tuesday 24 November 2020, 17:00
Venue: Webinar
Abstract: We will present the precise late-time asymptotics for scalar fields on both extremal and sub-extremal black holes including the full Reissner-Nordstrom family and the subextremal Kerr family. Asymptotics for higher angular modes will be presented for all cases. Applications in observational signatures will also be discussed. This work is joint with Y. Angelopoulos (Caltech) and D. Gajic (Cambridge)
Recording: The recorded talk can be found here.

Quantum fluctuations and instantons

Speaker: Eliezer Rabinovici
Institution: Hebrew University of Jerusalem
Time: Tuesday 01 December 2020, 13:00
Venue: Webinar
Abstract: After a review of Coleman's work on the fate of a false vacuum we turn to consider the decay involved in circumstances where the methods suggested by Coleman run into difficulties. We find that in those cases quantum fluctuations play a crucial role. We suggest that for a certain range of parameters a classical picture can be maintained. This involves identifying ultra-violet and infra-red cut offs whose value is induced by the parameters of the potential. In the presence of these cutoffs one of Coleman's boundary condition needs to be reconsidered. This allows to obtain a new class of O(N) invariant solutions which lead to acceptable results for the decay constant of the false vacuum.
Recording: The recorded talk can be found here.

Comments on Chiral Fermions

Speaker: David Tong
Institution: University of Cambridge
Time: Tuesday 08 December 2020, 13:00
Venue: Webinar
Abstract: A well known piece of folklore says that it's not possible to give fermions in a chiral gauge theory a mass without breaking the gauge symmetry. This folklore is wrong. I'll demonstrate this by showing how to give one generation of fermions in the Standard Model a mass without breaking electroweak symmetry.
Recording: The recorded talk can be found here.

Quantum Extremal Islands Made Easy

Speaker: Robert Myers
Institution: Perimeter Institute
Time: Tuesday 15 December 2020, 17:00
Venue: Webinar
Abstract: We illustrate the appearance of quantum extremal islands in holographic setups in higher dimensions. We emphasize the close parallel with the behaviour of holographic entanglement entropy in more familiar settings.
Recording: The recorded talk can be found here.